If you look at the order it will state something of this sort:
for a ptt of 80-109 sec leave rate unchanged (all you have to do is check labs). If first therapeutic dose, recheck in 6 hours, if second, recheck in a.m.
for a ptt of 60-79 increase rate by 170 units/hr (1.7 ml/hr), recheck ptt in 6 hours. (numbers vary)
Of course, for higher dose, you will see a decrease. ml/hrs are set by md's according to weight?? I suppose. Some orders ask you to hold it for 1/2 h (if ptt is really elevated). Some ask you to bolus patient to get to a therapeutic level faster.
Remember that a heparin therapeutic level is usually 2.0-2.5 higher than a normal levels.
People on heparin drips need to be monitored for bleeding. Usually cardiac patients, so full cardiac assessment, etc.
All you have to do is look at the labs and follow the order (and question the order if it is inappropriate). Not too long ago a provider did not enter the parameters in the sliding scale, a secretary transcribed the order, and an RN signed it! When I got the patient, heparin had not been hanged yet. So you bet I called!